In this paper, a novel compact quintuple-mode UWB bandpass filter (BPF) with sharp rejection skirt and wide upper-stopband performances is realized using stub-loaded multiple-mode resonator (MMR). The proposed resonator is formed by attaching two pairs of circular impedance-stepped open stubs in shunt and a pair of short-circuited stubs to high impedance microstrip line. By simply adjusting the radius of circular impedance-stepped open stubs and the lengths of short-circuited stubs, the first five resonant modes of the resonator can be roughly allocated within the 3.1--10.6 GHz UWB band meanwhile the high resonant modes in the upper-stopband can be suppressed. The short stubs in pairs can generate two transmission zeros near the lower and upper cut-off frequencies, leading to sharper rejection skirt outside the desired passband. Finally, a quintuple-mode UWB BPF is designed and fabricated, and the measured results demonstrate the feasibility of the design process.
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